1. Field of the Invention
The present invention relates to the field of electro-mechanical devices, and more particularly to stepper motors and the like. 2. Prior Art
Step motors, or as they are often called, stepper motors, are motors which are primarily characterized by their step-wise motion resulting from the sequential excitation thereof through the plurality of excitation states of the motor. Such devices characteristically provide a relatively high torque and limited and controllable motion, which is specifically desired in some applications. In recent years the interest in and the application of such devices has grossly expanded, as such devices are ideal in applications requiring closely controlled motion, often sought in automatic equipment, and have excitation and response characteristics easily interfaced with digital equipment to provide an electro-mechanical drive for computer control. Accordingly, various types of stepper motors are well known in the prior art.
One common type of stepper motor is referred to as a variable reluctance stepper motor, having a wound stator and a soft magnetic rotor. One specific configuration for such devices uses a plurality of wound stators separated axially along the axis of a multiple pole (salient pole) soft magnetic rotor, with the stators being rotated slightly, one with respect to the other, so that each stator may incrementally advance the rotor with respect to the previous stator. By way of specific example, for a configuration having three such stators, each stator would be rotated with respect to the other two by an angle equal to one-third of the pitch between adjacent rotor poles, so that sequentially cycling the excitation between the three stators will advance the rotor position by one rotor pole per excitation cycle. Such a configuration is relatively simple in conception and construction, and may be fabricated having substantially any reasonable number of phases. It has a disadvantage however, of providing a relatively long assembly of the "stack" of wound stators about the single rotor, and accordingly is relatively heavy and bulky for the torque provided. Such motors are described in a book entitled "Theory and Applications of Step Motors" by Benjamin C. Kuo, a West Publishing Company publication (1974).
Another type of variable reluctance motor is described in U.S. Pat. No. 2,249,029. The motor described therein is similar to the motor described hereabove, though only a single wound stator is provided. That stator is divided into segments each having a plurality of poles, with alternate segments effectively representing alternate stators of the hereinbefore described variable reluctance stepper motor. Additional examples of such devices include the disclosures of U.S. Pat. Nos. 3,439,200; 3,535,604; and 3,509,392. Generally speaking, such devices use a single coil to excite all poles within a specific pole group, resulting in adjacent poles within the pole group having the same polarity.
Various types of linear variable reluctance motors are also known. Such devices may be fabricated by merely "flattening out" the pole configuration of a rotary variable reluctance motor at the air gap, and repeating the pole pattern over the particular length of drive required. An example of such a device includes that of U.S. Pat. No. 3,376,518. As shown in the foregoing patent, any such technique may also readily be extended to a second, typically an orthogonal, direction.
Further, variable reluctance motors utilizing some form of bias magnet, e.g., permanent magnet, are also known. By way of example, in the book hereinbefore referred to, a device identified as the CYCLONOME step motor is described wherein a control coil controls the flux pattern of a permanent magnet in such a way as to controllably advance the rotor as a result thereof. Similarly, in U.S. Pat. No. 3,457,482, a permanent magnet is utilized as a bias magnet, with control windings controlling the permanent magnet flux path at the pole faces adjacent the soft magnetic rotor. In these devices it is important to note that the permanent magnets are used as bias magnets, with the control windings being provided on the same magnetic element, e.g., the bias magnets as well as the windings form the "stator", operating in conjunction with a salient pole soft magnetic "rotor".
In addition to the foregoing devices, permanent magnet stepper motors are also well known. One type of such motor, also described in the hereinbefore referred to book, utilizes first and second axially displaced salient pole soft magnetic rotor elements having an axially disposed permanent magnet therebetween so that all salient poles on one rotor element are of one polarity, and all salient poles on the other rotor element are of opposite polarity. A pair of wound stators are provided, each aligned with one of the rotor elements. Each stator has a plurality of pole groups with a single winding for each pole group, and with all poles in all pole groups having the same pitch as the adjacent rotor element. As before, one stator is rotated with respect to its adjacent rotor element in comparison to the other stator-rotor element combinations. The back iron coupling the two stators allows the windings for one or more pole groups in a particular stator to be excited with one plurality, and a corresponding pole group on the other stator to be excited with the opposite plurality.
In essence, the foregoing device is similar to the device of U.S. Pat. No. 2,249,029, with the permanent magnet providing a biasing of the rotor element, and with the second stator rotor element combination providing a duplicate motor for the second pole of the bias magnet. Because of the bias magnet however, the torque provided by any one set of coils is dependent upon the polarity of the excitation provided, and accordingly, the number of excitation states is twice the number of coil sets.
U.S. Pat. No. 3,978,356 discloses a self starting synchronous motor with permanent magnet rotor using a rotor having equally spaced poles of opposite polarity and a nonsymetrical one-piece stator having pole groups each with the same pitch as the rotor. Some of the pole groups are mechanically offset with respect to the other pole groups by a mechanical offset angle apparently selected so as to enhance the self starting characteristics (see also U.S. Pat. No. 4,031,419 of the same inventor disclosing related subject matter). Finally U.S. Pat. No. 3,344,325 discloses a permanent magnet motor having a rotor of specified numbers of equally spaced opposite polarity rotor poles and a stator comprising individual elements each having two poles and a winding therebetween to polarize the poles with opposite polarity. This disclosure is specifically limited to specific rotor and stator configurations as described therein.